Down in the market area there's a pinned thread for people asking the market value of various creatures. Perusing there would give you a good estimate of going prices. Also check Master Belmos's profile - he has a huge market of creatures, though I think his prices tend to run high?

Last I recall donation pets and giftborns tend to go at the highest rate; donis start at 50k per shard when released and then go up or down after retirement based on popularity. Giftborns can be rarer than many donis, especially since there haven't been new ones for years now, not everybody got them when they were issued, and those that did generally got only one. Tinsel wyrms, coal wyrms, and the two pengus also have limited breedings, which really drives up the scarcity even for later generations. They were reissued in a grab bag a few years back, which helped, but the limited breedings still makes it hard.

NightmareFire wrote:Are we allowed to do cross-site trading, like say Felisfire or FlightRising for Magistream creatures, or vice versa?

From the Rules thread

No Off-site Trading

It is not allowed to trade Magistream gold or creatures for credits or creatures from other sites. Likewise, selling Magistream gold or creatures for real money is forbidden as well due to the high scamming risk.